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J Med Libr Assoc. Oct 2007; 95(4): 442–445.
PMCID: PMC2000776

Comparing test searches in PubMed and Google Scholar

Mary Shultz, MS, AHIP

INTRODUCTION

Google Scholar has been met with both enthusiasm and criticism since its introduction in 2004. This search engine provides a simple way to access “peer-reviewed papers, theses, books, abstracts, and articles from academic publishers' sites, professional societies, preprint repositories, universities and other scholarly organizations” [1]. An obvious strength of Google Scholar is its intuitive interface, as the main search engine interface consists of a simple query box. In contrast, databases, such as PubMed, utilize search interfaces that offer a greater variety of advanced features. These additional features, while powerful, often lead to a complexity that may require a substantial investment of time to master. It has been observed that Google Scholar may allow searchers to “find some resources they can use rather than be frustrated by a database's search screen” [2]. Some even feel that “Google Scholar's simplicity may eventually consume PubMed” [3].

Along with ease of use, Google Scholar carries the familiar “Google” brand name. As Kennedy and Price so aptly stated, “College students AND professors might not know that library databases exist, but they sure know Google” [4]. The familiarity of Google may allow librarians and educators to ease students into the scholarly searching process by starting with Google Scholar and eventually moving to more complex systems. Felter noted that “as researchers work with Google Scholar and reach limitations of searching capabilities and options, they may become more receptive to other products” [5].

Google Scholar is also thought to provide increased access to gray literature [2], as it retrieves more than journal articles and includes preprint archives, conference proceedings, and institutional repositories [6]. Google Scholar also includes links to the online collections of some academic libraries. Including these access points in Google Scholar retrieval sets may ultimately help more users reach more of their own institution's subscriptions [7].

While its advantages are substantial, Google Scholar is not without flaws. The shortcomings of the system and its search interface have been well documented in the literature and include lack of reliable advanced search functions, lack of controlled vocabulary, and issues regarding scope of coverage and currency. Table 1 summarizes some of the reported criticisms of Google Scholar.

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Table 1 Criticisms of Google Scholar

Vine found that while Google Scholar pulls in data from PubMed, many PubMed records are missing [20], and that Google Scholar also lacks features available in MEDLINE [12]. Others have noted that Google Scholar should not be the first or sole choice when searching for patient care information, clinical trials, or literature reviews [23,24]. Thorough review and testing of Google Scholar, being an approach similar to that used to evaluate licensed resources, is necessary to better understand its strengths and limitations. As Jacso states, “professional searchers must do sample test searches and correctly interpret the results to corroborate claims and get factual information about databases” [18]. This paper compares and contrasts a variety of test searches in PubMed and Google Scholar to gain a better understanding of Google Scholar's searching capabilities.

METHODOLOGY

Ten searches were performed in PubMed using a variety of available search features. The searches were repeated in Google Scholar to approximate a user's approach to those same topics in that search engine. The searches, performed between August and September 2006, were by topic, author, title, journal name, and/ or combinations of those fields (Appendix online). Topics included iron-deficiency anemia, bupropion for smoking cessation, and articles by specific authors in specific journals. The topics selected were loosely based on questions received during reference transactions or were previously developed for use during instruction.

For each search, the citations received via Google Scholar and PubMed were examined to determine a variety of characteristics including format, date, Medical Subject Headings (MeSH) where appropriate, uniqueness, duplications, and full-text availability from the author's institution.

Most searches were narrowed by date to produce sets of a reasonable size to allow comparison of unique items retrieved by each system. The search results were analyzed to determine possible reasons for the retrieval of unique items in each resource and to gather information on the general features of the Google Scholar results.

RESULTS

In eight of the ten searches, Google Scholar returned larger retrieval sets than PubMed (Table 2). Table 3 illustrates the characteristics of the items retrieved by Google Scholar, and Table 4 provides information on PubMed retrieval sets. Most items retrieved by Google Scholar were journal articles (Table 3). Items in other formats included: 9 books, 11 book reviews, 2 Web pages, 1 subject index listing, 1 thesis, 1 newsletter item, 1 bibliography, 4 author replies, 1 annual meeting abstract, and 1 draft document. These results yielded few gray literature items.

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Table 2 Number of retrieved items
Table thumbnail
Table 3 Characteristics of Google Scholar results
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Table 4 Characteristics of PubMed results

The main title link in Google Scholar citations was used to determine if full text was found. Full text was available in 46.96% (116/247) of the total citations retrieved. In most cases, it was assumed that full-text access was based on the institutional subscriptions available to the author of this study. Some items retrieved might have been freely available. In 22.67% (56/247) of the results, the Google Scholar citation was simply a link out to a PubMed record. As shown in Table 4, nearly half (48.98%; 72/147) of PubMed citations provided full-text access through the author's institution.

The unique items retrieved by each interface were examined to determine why they were missed by the other system. Across all searches, Google Scholar retrieved a total of 247 citations, 125 (50.61%) of which were unique to Google Scholar. Analysis revealed the following characteristics:

  • Thirty-two items (12.96%) retrieved by Google Scholar were formats other than journal articles.
  • Some unique Google Scholar items (10 items, 4.05%) appeared in journals not indexed by PubMed.
  • Google Scholar covered a wider date range and returned 4 items (1.62%) older than 1950 that were not in PubMed.
  • Google Scholar retrieved items based on its ability to search the full text of many articles rather than solely on citation data.

PubMed retrieved a total of 147 citations across all searches, and, of these, 46 (31.29%) were unique.

DISCUSSION

Assumptions of search engine performance based purely on retrieval quantities can be misleading without closer investigation of the results. For example, Table 2 shows that many of the searches returned quantities that were close in numbers. In search #1 (dietary supplements as a treatment for iron deficiency anemia), PubMed returned twenty-five citations, while Google Scholar returned twenty-six citations. However, only four citations were common to both systems. In search #2 (Mobius syndrome), Google Scholar returned eleven citations, while PubMed found ten citations but with an overlap of only two citations retrieved by both systems.

Terminology was observed to be a major factor affecting retrieval and the ability of both systems to return unique items. Some unique items retrieved by Google Scholar were off topic. These “false hits” appear to be related to Google Scholar's full-text searching along with a lack of controlled vocabulary. For example, the purpose of search #7 was to find articles on the topic of “wine” that appeared in the New England Journal of Medicine. Google Scholar retrieved eight items where the word “wine” appeared in the full text but was not the main topic of the article, in one case, retrieving an article where the authors acknowledge a colleague with the surname Wine. Google Scholar also returned items that contained the search terminology but did not match the intention of the search. In the search for information about dietary supplements in the treatment of iron deficiency (search #1), Google Scholar returned some citations about high iron stores rather than deficiency (Table 5 online). Google Scholar searches for a word or sequence of letters and not the concept or meaning.

The complete citations for all unique items retrieved by PubMed were examined. One possible explanation why Google Scholar failed to retrieve the same items was that many were indexed under the appropriate MeSH term, although the search phrase might not have appeared in the title or abstract. For example, search #9 was designed to retrieve articles by Visek about the topic of ammonia. While ammonia was not searched specifically as a MeSH term, PubMed automatically mapped it to MeSH. Of the unique citations retrieved by PubMed, some were indexed under ammonia although this term did not appear in the citation (Table 5 online). While Google Scholar offers the ability to use a tilde (~) to retrieve alternative terminology, this ability does not provide the control that subject headings do.

CONCLUSION

Performing a direct and exact comparison between searches in Google Scholar and PubMed is not possible as the systems function in very different manners. For example, PubMed searches a well-defined set of journals, while Google Scholar includes resources beyond journals and the exact scope of coverage is not extensively described. Because the systems are not searching identical data, the results are often different.

Although these two systems are difficult to compare, it is still important to explore the differences between them. Librarians should understand the strengths and weaknesses of Google Scholar and be prepared to explain them to their users [14]. It may also be wise to consider including Google Scholar in bibliographic instructional sessions and to convey how it compares to other search interfaces [11]. For example, Google Scholar does not offer the number and extent of special searching and limiting features available in PubMed. However, Google Scholar provides some advantages in that it is an easy place to begin a search to find an initial retrieval of possibly worthwhile articles. It also offers searchers the ability to find citations to older items that they would miss if they use only PubMed. Additionally, Google Scholar has the potential to provide access to the gray literature. This increased access to a part of the biomedical literature, which can be difficult to search, may have implications for the public health field [25].

One of the most advantageous features of searching PubMed is the ability to utilize the MeSH vocabulary, as Google Scholar does not currently implement controlled vocabulary searching mechanisms. MeSH provides a powerful method of narrowing results and homing in on what the searcher needs. PubMed also offers substantially more features that allow searchers to narrow their retrieval to citations from clearly identified sources, as detailed in NLM's List of Journals Indexed for MEDLINE and List of Serials Indexed for Online Users [26]. The problem faced today by searchers is not a lack of information but rather an overload of information. For a researcher conducting human studies, writing a dissertation, finding information pertinent to patient care, or conducting an in-depth literature review, Google Scholar does not appear to be a replacement for PubMed, though it may serve effectively as an adjunct resource to complement databases with more fully developed searching features. It is important to note that both PubMed and Google Scholar are often upgraded with new features or with intended improvement of existing functions. It may be worthwhile to repeat this study in one or two years to determine if further refinements have improved their performance.

Supplementary Material

Appendix:
Table 5:

Acknowledgments

The author thanks the following individuals who offered invaluable advice and support: Pauline Cochrane, Robin Beck, Sandra De Groote, AHIP, Victoria Pifalo, and Ann Carol Weller.

Footnotes

Supplemental Table 5 and an appendix are available with the online version of this journal.

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